Protein Clusters database records that are linked to the proteins involved in the biosystem. The association between the proteins and biosystems was made using the method described in the data processing section of the BioSystems help document. The Protein Clusters database contains both curated and uncurated clusters of proteins (from four major groups: prokaryotes, bacteriophages and the mitochondrial and chloroplast organelles) that are grouped by sequence similarity.

Conserved domains that are specific hits to the proteins involved in biosystems, representing high confidence functional annotations. The association between the proteins and biosystems was made using the method described in the data processing section of the BioSystems help document.

HomoloGene records that are linked to the proteins involved in the biosystem. The HomoloGene records list putative orthologs of the genes that coded the proteins. The association between proteins and biosystems was made using the method descibed in the data processing section of the BioSystems help document.

Records from the Online Mendelian Inheritance in Man (OMIM) database that are linked to the Entrez Gene records associated with a BioSystem. The association between genes and biosystems was made using the method descibed in the data processing section of the BioSystems help document.

Biosystems share at least one identical protein sequence from the same source organism. In other words, the shared protein is in the same protein identity group ("PIG," a group of proteins that are identical in sequence) and has the same Taxonomy ID.

BioAssays whose protein targets are involved in biosystems. The association between the proteins and biosystems was made using the method described in the data processing section of the BioSystems help document.

3D macromolecular structures of proteins involved in biosystems. The association between the proteins and biosystems was made using the method described in the data processing section of the BioSystems help document.

The source organisms of the biosytems, whose Taxonomy IDs were provided in the biosystem records deposited by the source database. For additional information, see the data processing/taxonomy section of the BioSystems help document.